Transistors work as amplifiers, delivering larger amounts of juice when triggered by a relatively small amount of voltage. One advantage of this is that you can use it to detect slightly conductive substances (e.g., tap water) and trigger an effect thereby.

Garrett used a TIP-120 Darlington transistor, a capacitor, a resistor, and a heat sink to create a sensor that can detect when its electrodes have been immersed in a conductive liquid.

When the electrodes contact the liquid liquid, current flows from the power supply to the Darlington’s base pin, triggering current flow from the collector to the emitter.